The Alley Tramp (1968)

Sixteen year old Marie stumbles upon her parents making love, which awakens her latent sexuality. She begins a torrid affair with her third cousin Philip. Her parents cluelessly pursue their own illicit affairs while their daughter delves deeper into her own sexual experimentation. When the school principal makes the Barkers aware that their daughter is cutting classes, they purchase a partial clue and try to help their daughter.